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Divide and rule

By Bob Holmes in Santa Cruz

LAND plants may owe their evolutionary success to the accidental duplication
of a gene early in their history, say scientists in the US and Germany.

The gene in question codes for actin, a key component of the internal
skeleton of cells. Like animals, higher land plants have many different actin
genes, and the different versions of the protein have different properties. This
diversity helps cells perform specialised functions, an essential step in the
evolution of complex multicellular organisms.

For years, however, researchers have debated whether the variety of actin
genes was what enabled land plants to diversify, or whether the genes
diversified later.

Now researchers believe they have evidence to support the first theory.
Debashish Bhattacharya of the University of Iowa in Iowa City and his colleagues
sequenced the actin genes of three primitive land plants, and several green
algae. The green algae have a single version of the gene, they found, as does
Mesostigma viride, a single-celled organism that represents the lowest
branch on the evolutionary tree leading to land plants. In contrast, all the
other primitive land plants they studied have at least two actin
genes—evidence that the gene was duplicated around the time the lineage
evolved.

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Higher up the tree, more duplications appear, Bhattacharya told a meeting of
the International Society for Evolutionary Protistology in Flagstaff, Arizona,
earlier this month. This suggests that gene duplication and divergence were key
events in the origin of complexity in land plants, he says.

The evidence fits well with other data, says Richard Meagher, an expert on
the actins of higher plants at the University of Georgia. But to clinch the
point, he says, Bhattacharya’s group must also show that the plant uses its two
or more actin genes in different contexts. Bhattacharya says that this
investigation is now under way.